1. Field of the Invention
The present invention relates to a vehicle brake system having liquid pressure supply units for outputting liquid pressure in response to a braking operation, and a wheel cylinder for braking a wheel upon receiving the liquid pressure from the respective liquid pressure supply unit.
2. Description of the Prior Art
A typical liquid pressure supply unit in a vehicle brake system comprises a vacuum or an electric booster, automatically actuated based on the signal output from an obstacle sensor for detecting an obstacle in the path of the vehicle or actuated by a driver, and a master cylinder which is driven by the booster and supplies liquid pressure to a wheel cylinder (e.g., Japanese Patent Publication No. 8-18548(1996)).
However, there still exists the problem of causing an output delay of liquid pressure from the master cylinder, the problem arising from the time required to fill up an operating chamber of the booster with the pressurized operating liquid at the initial stage of braking operation, and from the ineffective stroke of the piston required to close the relief port of the master cylinder and so on.